1. Field of the Invention
This invention pertains generally to devices and methods for metal welding, and more particularly to a welding system and method which provides for quick, easy and accurate vertical welds using a light weight, portable welding fixture and a distributed control system which allows an operator to control arc voltage, electrode oscillation, wire feed rate, seam tracking, and other functions during welding operations.
2. Description of the Background Art
When welding metal items together using arc or gas welding techniques, horizontal welding has traditionally been easier and less expensive to carry out than vertical welding. While welding metal substrates with conventional welding methods or plates together in a horizontal position, gravity assists in keeping the molten weld puddle in place and facilitates the formation of high quality welds. With vertical welding of metal substrates, the molten weld puddle is much more difficult to control, and the weld is correspondingly more expensive and time-consuming to perform. For this reason, structural steel fabricators go to great lengths to position metal substrates in a horizontal relationship during welding and thereby avoid vertical welds.
The problems associated with making vertical welds are particularly evident in the welding of "stiffener" plates into steel I beams or H columns for use in building construction. These stiffeners are used to transfer the moment load through a vertical column when a horizontal column is welded to it. The welding of stiffeners into structural beams is one of the most common welding operations and consumes thousands of man-hours per year for a typical structural steel fabricator. The stiffener plates are welded to the web and flanges of a column in a position which is normal to the web and flanges of the column. Thus, the weld connecting the stiffener plate to the web is at a right angle to the welds which join the stiffener plate to the flanges, and to complete all of the welds, the steel fabricator must either continually reposition the heavy steel beam to maintain a horizontal position for each weld, or must carry out difficult vertical welds.
Heretofore, the most common method of welding stiffeners into beams or columns has been through use of conventional "flux-cored" welding wire methods. Flux-cored welding generally involves filling weld joints with weld metal from a flux cored welding wire. The wire is made from a flat metal strip which is drawn into a hollow tube, filled with a powdered flux material, and rolled on a spool. During welding, the wire is unwound from the spool and fed through a flexible cable or conduit by a wire feeder device to a welding gun. When an operator presses the trigger on the gun, the wire is fed out of the gun and strikes an arc on the parent material to be welded. The arc energy melts the wire and parent material to form a homogeneous weld of fused wire and parent material.
In order to properly weld stiffeners in place on columns using flux-cored welding, the stiffener plate and weld joint must be properly prepared so that the weld will meet the AWS (American Welding Society) code requirements. Generally, the stiffener is first cut from a standard piece of mill plate and then bevel cut on three sides and ground clean to remove any mill scale. Back-up bars, which retain molten metal in place during welding, are then prepared for a fit-up operation wherein the stiffener plate is carefully positioned relative to the column. The person carrying out the fit-up operation must weld the stiffener and backup bars to the column such that a constant 3/8 inch gap is maintained between the stiffener plate and the parent material of the column. If the gap is too narrow, the stiffener must be ground until the proper gap is achieved. If the gap is too wide, the weld will require more metal (and thus more weld passes) to fill. Many welding or construction codes require that the backup bars be removed after the stiffener has been welded in place. Such removal is difficult and expensive, and generally requires gouging out the backup bars with a carbon arc, followed by additional weld passes to fill in the gouged areas.
Small structural steel fabricators generally weld stiffeners into columns using flux-cored wire welding while the columns are horizontally positioned between two upright supports, with the columns being continually flipped or repositioned for each weld to avoid vertical welding. Since the columns generally are very heavy, an overhead crane is used to lift the columns for repositioning. This process is very time consuming and expensive. Additionally, multiple weld passes are required to fill each weld joint, with thicker stiffener plates requiring more weld passes. After each weld pass, the operator must stop and chip off the slag covering the weld before the next weld pass. If any defects occur, the defect must be gouged out with a carbon arc and re- welded.
Larger structural steel fabricators sometimes use "pit welding" or "platform welding" for installing stiffeners, wherein columns are positioned vertically so that all three sides of the weld joint are in a horizontal or flat "hog-trough" position. Since the column is vertical, the stiffener is horizontal and the welds on all three sides are made in the horizontal position. This arrangement also allows a much larger puddle during welding, and requires fewer weld passes to fill each weld joint. However, the handling and positioning of vertically oriented columns is difficult and requires an overhead crane and the use of a pit and/or platform, thus requiring a large amount of work space. Further, the location of the welding operation is generally not at ground or floor level when using pit or platform welding techniques, and can require the welding operator to be awkwardly or precariously positioned on a platform or ladder during the welding operation.
A vertical welding technique known as "electroslag" welding (ESW) has been developed to overcome the difficulties associated with repositioning columns or other heavy substrates in order to permit horizontal welds. The electroslag method generally involves bringing the ends of two vertically-oriented plates or substrates together such that a 3/4 inch to one inch gap remains between the ends of the plates. Copper welding shoes are then placed on each side of the gap to form a vertical channel or cavity between the plates and welding shoes. This cavity is filled with weld metal by placing a steel guide tube into the cavity to feed welding wire into the channel. When the welding wire feeds out the bottom end of the guide tube, an arc is struck against the parent material and a molten puddle is formed. A granular flux material is sprinkled into the channel during welding, which melts to form a conductive slag. The arc is extinguished by the conductive slag, which remains molten due to the resistance to the electric current passing between the welding wire and the substrates. Heat generated by the resistance of the molten slag melts the welding wire and fuses the molten metal to the substrates to form the weld. The welding wire is continually fed into the weld while the guide tube is oscillated or reciprocated within the cavity, and the cavity is filled with molten metal to join the plates together. The guide tube is consumable and contributes to the weld metal. The copper shoes retain the weld puddle in place, and are removed when the weld is completed. A comprehensive description of electroslag welding is provided in the American Welding Society Welding Handbook, eighth edition, which is incorporated by reference.
While the electroslag process permits vertical welds, it has previously not met with much success due to the large amount of time required to set up prior to welding. Particularly, it is difficult and time consuming to position and secure the copper shoes about the gap between the substrates which are to be welded. In the case of electroslag welding of stiffeners onto columns, "L"-brackets generally must be cut and welded into place between the flanges and stiffener in order to support the copper shoes, with two L-brackets required for each weld. After the L-brackets are welded in place, steel wedges are pounded in place between the L-brackets and copper shoes to hold the shoes in position. When the weld is finished, the brackets must be removed.
Another drawback associated with conventional electroslag welding is that that the guide tube must be carefully positioned within the gap to be welded, which requires careful alignment of the welding head and welding oscillator mechanism. Incorrect alignment of the guide tube can result in contact of the guide tube with tone of the copper shoes during welding, causing a 500 Amp short which will generally destroy the (expensive) copper welding shoe and interrupt the welding operation. Any such interruption of an electroslag weld operation is very inconvenient and generally requires gouging out the incomplete weld and starting the entire operation over.
Still another drawback of conventional electroslag welding is that the molten flux puddle in the weld cavity can cause the welding wire to fuse to the bottom of the guide tube during welding, which prevents wire from feeding into the weld. The welding then must be interrupted, the copper shoes removed, and the weld area cleaned or ground down to allow set up for a new weld start. As noted above, the interruption of an electroslag weld in such a manner requires expensive and time consuming cleanup of the incomplete weld followed by starting the weld operation over again.
Welding controllers or control systems have been developed to facilitate electroslag welding by controlling wire feed rate and oscillation, but such controllers generally bulky and heavy, and typically provide for only one type of weld condition. If the weld condition varies during welding, defects may occur to the weld, or a catastrophic short against one of the copper welding shoes may occur. For these reasons, electroslag welding of stiffeners onto columns has not proved economical, and the welding industry has continued to use the flux-cored wire welding method. Further, previously known welding control systems have been based on centralized control architectures having a star topology. These control systems are generally not scaleable or adaptable to changing needs or different types of welding operations. Generally, the central processor board for such systems must be re-designed and modified to meet new requirements.
Accordingly, there is a need for a welding system and method which overcomes the drawbacks presently associated with the currently-used flux-cored wire welding and electroslag welding methods, which eliminates the need for frequent re-positioning of heavy steel columns or other substrates during welding operations, which allows quick and easy vertical welding with minimal set up time, which uses light weight, portable equipment, which prevents unwanted interruption of welding operations, and which provides a distributed control system to allow defect free welds under a variety of weld conditions. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies of conventional electroslag welding and flux-cored wire welding methods, and the drawbacks found generally in the background art.